COVER STORY

Atmospheric aerosols are tiny solid or liquid-covered particles suspended in the air. Aerosols (particulate matter) play a very crucial role in the dynamics of the atmosphere. Almost every weather event in the troposphere is mediated by aerosols, either directly or indirectly. Aerosols serve as nuclei for cloud con­densation and precipitation, and also control the tropospheric temperature by absorbing or scattering solar radiation.

However, these life-supporting aero­sols can become harmful when their natural equilibrium in the environment is disturbed. One such problematic aerosol is Black Carbon (BC). BC re­fers to carbonaceous, polycyclic organic aerosols found in the atmosphere and is commonly referred to as ‘soot’. BC is produced through incomplete combus­tion and consists primarily of polycyclic carbon structures with high molecular diversity. Although the terms ‘soot’ and ‘BC’ are often used interchangeably, soot is a general term. It refers to a com­plex mixture of particles and gases that are unwanted by-products of incomplete combustion, with BC being one of the key components

The Serbian physicist Tihomir No­vakov first used the term ‘BC’ in the 1970s when he discovered that aerosols contained fine particulate matter.

In contrast to other greenhouse gases present in our environment, BC is complex in nature and it is not a single molecule or gas. The term BC usually indicates a group of chemicals that sub­stantially absorb the solar radiation on earth. BC also includes light-absorbing organic carbon as well as elemental carbon. Although these aerosols have a relatively shorter lifetime in our atmo­sphere, they significantly impact climate directly as well as indirectly.

BLACK CARBON: CHARACTERISTICS AND SOURCES

BC exhibits significant variability in particle size, structure, and morphol­ogy. BC aerosols are often spherical, and their dynamic shape factors suggest that their particle morphology evolves dur­ing atmospheric ageing. Typically, BC aerosols have aerodynamic diameters ranging from 200 500 nm, with par­ticles consisting either predominantly of BC or BC internally mixed with aerosol components. However, to understand the optical properties and radiative forc­ing due to BC aerosols, one need to mea­sure the effective density and mass size distribution of BC in view of its dynamic behaviour. It is noteworthy that the freshly emitted BC aerosols are mainly hydrophobic in nature. However, the acquisition of soluble coating materials during atmospheric aging increases their hygroscopicity, thereby influencing their ability to act as cloud condensation nu­clei affecting their atmospheric longev­ity. BC has complex optical properties as well as complex morphology. Fresh BC typically aggregates as small spherical monomers, while the atmospheric ag­ing can lead to further particle restruc­turing, resulting in more compact and uniform morphologies. As a result, BC particles show substantial variability in homogeneity and refractive indices.

Typically, BC is released into the at­mosphere through incomplete combus­tion of biomass and fossil fuels and is fundamentally anthropogenic in origin. A wide range of activities contribute to global emissions, such as open burning of biomass (forest fire) (42%), burning of domestic biomass (18%), diesel engines used in transport (14%), industrial diesel engines (10%), industrial activities and energy production, typically from low-capacity boilers (10%), and residential coal burned (6%), among others.

Further, the relative contributions of BC sources vary from region to re­gion. For instance, in Western Europe, traffic-related emissions appear to be the primary contributor, as elevated BC concentrations are frequently observed in areas with intense motorised traffic and major roadways, but in South Asia, a majority of soot emission is caused by the biomass combustion for cook­ing purposes, and in East Asia, the combustion of coal for residential and industrial purposes seems to contrib­ute more. Whereas the United States contributes more from industrial and vehicular emissions. There are many countries located in high-emitting zones that neither have proper measurements nor have stringent regulations regarding emissions. Many developing countries primarily lean on open uncontrolled biomass burning and the continued use of conventional energy sources (such as wood, coal, and kerosene) for normal day-to-day activities such as cooking, heating, and lighting.

ROLE OF BC IN GLOBAL WARMING AND CLIMATE CHANGE

An important characteristic of atmo­spheric BC aerosols is their strong abil­ity to absorb terrestrial solar radiation across the entire wavelength spectrum and re-emit it as low-frequency radia­tion, thereby warming the atmosphere.

In particular, in areas with high BC concentrations, the warming of both the atmosphere and the Earth’s surface is intensified which results in disrupted weather patterns and natural ecological cycles. In addition, BC is often released alongside other hazardous air pollutants including organic carbon (OC), carbon monoxide (CO), volatile organic com­pounds (VOCs), and other toxic organic carbon (OC) which all together signifi­cantly contribute to the degradation of air quality.

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BC particles significantly alter the regional meteorology and normal weather patterns that are vital for agri­culture and human well-being or health. With the blocking of the sunlight by BC and thus dimming of the Earth’s surface, the evaporation rates get lower which ultimately disrupt cloud formation. Fur­ther, when these particles settle on plant leaves, they cause the surface to heat up which can damage the cells, thus limit­ing the plant’s ability to absorb carbon dioxide through photosynthesis.

The present research studies on di­rect global radiative forcing of BC as­serts that after the carbon dioxide and methane gases, BC is either the second or third most prominent anthropogenic greenhouse factor. Though these stud­ies on radiative forcing present a com­prehensive understanding of the effects of climate change due to enhancing BC concentration in our environment, it is difficult to represent the strong and lo­calised impacts of BC aerosols in view of its highly uncertain or dynamic be­haviour.

Owing to its short atmospheric life­time, BC exerts highly concentrated warming effects in specific areas, cre­ating localised climate ‘hot spots’. To accurately assess the warming effects of BC, it is important to focus on its regional impacts, rather than relying on uncertain global estimates. The overall impact of BC on the regional climate is quite a difficult and complex problem to understand since it depends on sev­eral factors. These factors mainly in­clude the source of emission and the life period of BC in the atmosphere along with its interaction with other pollut­ants and clouds.

BLACK CARBON IN THE CRYOSPHERE

The Himalayan range, referred to as the water tower of Asia as well as the Third Pole, directly or indirectly feeds the majority of the world’s population. Due to the present scenario of climate change, it is a well-known fact that Hi­malayan glaciers are shrinking. Extreme weather events in these regions are in­creasing. Air pollution is one of the ma­jor contributing factors for disturbing the fragile Himalayan ecosystem. The presence of BC in the fragile Himalayan cryosphere is an alarming concern. By darkening the surface of the snow and ice, BC contributes to accelerating their melting rates and disturbs the energy equilibrium of the cryosphere. Research indicates that the BC concentrations in snow and ice are significantly higher in mid-latitude locations relative to polar areas. The BC accounts for around 20% of the drop in albedo during the glacier melt season.

As a result, the melting of glaciers has accelerated with diminished snow cover ultimately affecting available water resources and disturb­ing climate stability.

ENVIRONMENT AND HEALTH HAZARDS

Owing to nanoscale size of BC aerosol particles, they can infiltrate deep into lungs and enter the bloodstream, spreading toxic compounds, causing inflammation and oxida­tive stress throughout the body, which can worsen the pre-existing respiratory and cardiovascular problems.

Chronic exposure to BC is associated with numerous health problems including lung cancer, heart diseases, and respiratory infections. Infants are particularly vul­nerable since extended expo­sure to air pollutants, includ­ing BC, can severely impair lung development. Pregnant women also face increased risk of low birth weight and preterm birth. Chronic ex­posure, especially to BC, has also been linked to neurolog­ical effects, such as reduced cognitive function.

Images Courtesy: fightclimatechange.earth

Over 15% of deaths in Delhi-NCR were attributed to air pollution and the situation is more alarming today as Delhi is cur­rently grappling with extremely poor air quality with Air Quality Index (AQI) frequently in severe range. To safeguard public health particularly for children, the elderly and those with pre-existing health disorders, the lowering of toxic air pollutants is vital.

MITIGATION STRATEGIES

Due to the challenges in quantifying the warming impact of BC precisely, it still remains underrepresented in interna­tional climate policy. As of 2025, only nine nations have included explicit tar­gets for reducing BC emissions in their Nationally Determined Contributions (NDCs) in order to fulfil their commit­ments, set forth in the Paris Agreement. Seven other nations have included BC in their overall climate goals.

Addressing the BC pollution can only begin with a clear understanding of its behaviour after release into the atmosphere. Preventing those emissions at their first source is the important next step. It is necessary to have a compre­hensive understanding of the numerous sources that contribute to the production of BC, as well as the numerous strategies that can be utilised to reduce these emissions. Further, lo­calised efforts to reduce BC emissions will provide the greatest benefits in terms of reducing regional warming and improving public health outcomes.

Urban air quality plans need to be developed to reg­ulate city-specific pollution sources, including road and soil dust, vehicular emis­sions, domestic fuel, commu­nity-generated waste com­bustion, building materials, and industrial operations. These initiatives establish time-bound targets and are supported by annually up­dated implementation strate­gies with detailed micro-level planning for effective imple­mentation of city plans.

Transitioning brick kilns to zig-zag technology aims to mitigate pollution levels and the transition of industrial units to piped natural gas for reduced emissions is further recommended.

Image Courtesy: iStockphoto.com/PytyCzech

Implementing initiatives that inform the public about the detrimental impacts of BC is a necessity for mitigat­ing emissions. It is important to engage communities in discussions and projects that promote the use of low-emission cooking fuels and technol­ogies. Disseminating information on BC and its effects on health and the environ­ment, using various media platforms, can play a vital role. Building partner­ships between public and private enti­ties, including enterprises and diverse organisations, should be undertaken to curb BC emissions. We must also en­courage policies that support the use of cleaner technology, while also reduc­ing reliance on traditional biomass for heating and cooking purposes. Applying these strategies can considerably raise public awareness, which in turn will en­courage more people to work towards lowering BC emissions and improving air quality.

INDIAN INITIATIVES FOR BC MONITORING AND MITIGATION

The government of India has imple­mented various measures to regulate BC emissions, including the flagship pro­gramme, the Pradhan Mantri Ujjwala Yojana by Prime Minister Narendra Modi, which promotes the use of clean cooking fuels in households. The metro rail network for public transport has been expanded to include additional cit­ies, introduction of cleaner and alterna­tive fuels, such as gaseous fuels (CNG, LPG, etc.), has led to the launch of a new initiative titled ‘Sustainable Alter­native towards Affordable Transporta­tion (SATAT)’. This initiative aims to establish 5,000 Compressed Bio-Gas (CBG) production plants and facilitate the availability of CBG in the market for utilisation.

The Central Sector Scheme for the Promotion of Agricultural Mechaniza­tion for in situ Management of Crop Residue in Uttar Pradesh, Haryana, Punjab, and NCR Delhi provides a 50% incentive for individual farmers on agri­cultural machines, and equipment for in situ crop residue management, and an 80% incentive for setting up Custom Hiring Centres. The Union Govern­ment is executing the National Clean Air Programme as a long-term, time-bound strategy at the national level to address air pollution comprehensively, aiming for a 40% decrease in particulate matter concentrations by 2025-26.

The Central Pollution Control Board (CPCB) has identified 131 cities that ex­ceed quality standards based on ambient air quality levels and population size of over one million. The ‘Clean Air Ac­tion Plans’ tailored to specific cities have been developed and are now in the pro­cess of implementation. Achieving the goal of a healthy and clean environment requires strict adherence to the enacted regulations.

CONCLUSION

BC, which is a transient air pollutant, degrades air quality and thus signifi­cantly contributes to global warming and climate change on earth. The BC particles present in the air can be in­haled with deep penetration into the human body leading to serious health risks. BC is therefore at the intersec­tion of climate change, environmental impact, and public health. The accu­mulation of BC particles on Himalayan glaciers substantially accelerates the melting of snow, thereby changing re­gional climatic conditions. This in turn affects the fragile Himalayan ecology in diverse ways and water availability for populations residing downstream. The targeted monitoring of BC concentra­tions along with the implementation of well-designed mitigation strategies is im­portant to minimise its harmful effects. Initiatives to reduce BC emissions and the use of cleaner combustion technolo­gies can not only function as a proactive measure to protect our environment and public health but also effectively lessen the adverse effects of climate change.

*Dr Chhavi Pant Pandey is Scien­tist D at Wadia Institute of Himala­yan Geology, Dehradun. She can be reached at chhavi@wihg.res.in.